1. Field of the Invention
The present invention relates to an image reading device and an image forming apparatus.
2. Description of the Related Art
Various techniques for an image reading device, image reading method, image forming apparatus or image forming method have been developed until now.
For example, Japanese Patent Application Publication No. 2002-107631 discloses a reading optical system including at least a lens group that is rotationally symmetric with respect to its optical axis and at least one free-form surface mirror and projecting a picture of an original copy onto a one-dimensional image-capturing device in an image reading device, characterized in that the shape of a reflection surface of the free-form surface mirror is symmetric with respect to a plane including the intersection of the reflection surface of the free-form surface mirror with the optical axis of the lens group and being perpendicular to the one-dimensional arrangement direction of the image-capturing device and the shape of the reflection surface of the free-form surface mirror is asymmetric with respect to a plane including the normal line of the free-form surface mirror at the intersection and being parallel to the one-dimensional arrangement direction of the image-capturing device.
In the reading optical system disclosed in Japanese Patent Application Publication No. 2002-107631, the free-form surface mirror is used in addition to the lens group, whereby the number of lenses included in the lens group is reduced. In a practical example disclosed in Japanese Patent Application Publication No. 2002-107631, a free-form surface mirror having a positive power is not arranged at an object side but at an image side, whereby a portion of aberration that may not sufficiently be corrected by only the lenses is corrected by the mirror.
However, it may be difficult to obtain a wide angle reading optical system because no intermediate image is formed at the object side, and an object distance for the reading optical system may be increased. In a practical example disclosed in Japanese Patent Application No. 2002-107631, the distance from an object to an image (object-to-image distance) is 496 mm for a reading optical system with a reduction magnification of 0.16535 at an object height of 150 mm and thus the overall length of the reading optical system has been increased. Accordingly, the thickness of an apparatus containing the reading optical system is also increased, whereby the whole size of the apparatus is increased. Then, even if the long optical path between an object and the lens group is folded by a plane mirror(s) in order to reduce the size of the apparatus, the number of folds (number of plane mirrors) is increased, whereby the costs for processes of production, arrangement and positional adjustment of the plane mirrors are increased.
Furthermore, the free-form surface mirror is arranged in the optical path between the lens group and an image, whose optical path length is short, namely, the optical path at reduction side (image side). Therefore, when an image-capturing device arranged at the reduction side of the reading optical system is installed, only a small space may be provided for positional adjustment of the image-capturing device and the positional adjustment of the image-capturing device may be difficult.
Furthermore, Japanese Patent Application No. 05-003528 discloses an image reading device in which an optical head including a one-dimensional reading element and optical imaging means opposing to the reading element are arranged so as to oppose to a two-dimensional original copy of a picture and the original copy of a picture and the optical head are moved relatively, characterized in that an concave mirror is used as the optical imaging means.
For a reading optical system of an image reading device disclosed in Japanese Patent Application Publication No. 05-003528, only a concave mirror that generates no chromatic aberration is used as the optical imaging means. Because imaging is conducted by only one concave mirror having a power in a practical example disclosed in Japanese Patent Application No. 05-003528, it may be difficult to correct an aberration except chromatic aberrations. Therefore, it may be difficult to obtain a wide angle reading optical system whereby an object-to-image distance of the reading optical system may be increased. Accordingly, the thickness of the image reading device is increased, whereby the size of the image reading device is increased.
Moreover, Japanese Patent Application Publication No. 2004-109793 discloses an image reading device including an imaging lens for imaging picture information of an original copy and a solid state image-capturing device for reading the picture information, characterized in that a mirror having an anamorphic surface is arranged in the optical path between the imaging lens and the solid state image-capturing device.
In the image reading device disclosed in Japanese Patent Application No. 2004-109793, the mirror having an anamorphic surface is arranged in the optical path between the imaging lens and the solid state image-capturing device, namely, at an image side, whereby a portion of aberration that may not sufficiently be corrected by the lens is corrected by the mirror, similarly to the reading optical system disclosed in Japanese Patent Application Publication No. 2002-107631, in order to correct the difference between the imaging positions of the imaging lens in its main scanning direction and sub-scanning direction.
However, it may be difficult to obtain a wide angle optical system because no intermediate image is formed at an object side, whereby an object-to-image distance for the optical system may be increased. Therefore, the thickness of an image reading device including the optical system may be increased whereby the whole size of the image reading device may be increased.
Meanwhile, the inventors have found that a comparatively wide angle optical system having a comparatively small object-to-image distance is provided and a comparatively compact image reading device in which the number of a fold(s) of an optical path of an optical system is comparatively small and a size of its housing is comparatively small is provided by using such an optical system.